1. Field of the Invention
This invention relates to improvements in an optical system capable of maintaining the position of an object or its image constant even if the distance between the object and the image varies and also relates to applications thereof.
2. Description of the Prior Art
An optical system is known in which the system between two lenses is constructed into a parallel optical system and the distance between an object and its image can be changed by moving the two lenses relative to each other. That is, as shown in FIGS. 1A and 1B of the accompanying drawings, if an object P is disposed on the forward focus of a first positive lens L.sub.01, the image P' of the object P is formed on the rearward focus of a second positive lens L.sub.02 and, if the distance between the two lenses is changed, the distance between the object P and the image P' can be changed arbitrarily. The image magnification is a predetermined value determined by the ratio of the focal lengths of the two lenses, and it is also well known that the position of the object or the image can be fixed if only one of the lenses is moved. In such optical system, however, when the spacing between the two lenses is equal to the sum (f.sub.1 +f.sub.2) of the focal lengths of the two lenses as shown in FIG. 1A, a light ray parallel to the optical axis which enters the second positive lens L.sub.02 from the image P' side is still parallel to the optical axis, as indicated by broken line in the Figure, even after it leaves the firt positive lens L.sub.01, whereby so-called telecentricity is maintained on both sides, whereas if the spacing between the two lenses changes as shown in FIG. 1B, the telecentricity on the object P side can no longer be maintained. That is, although the distance between the object and the image can be changed arbitrarily by changing the spacing between the two lenses, the conjugate relation of the pupil of this optical system cannot be maintained constant. Accordingly, it is only for a particular spacing between the two lenses that a telecentric illuminating light can be supplied where the object P is to be coaxially illuminated, and in the other cases, telecentric illumination, i.e., Keller illumination, could not be accomplished unless the light source itself was moved.